Thursday, March 16, 2017

Shortening a Silca Frame Pump

Despite the popularity of mini pumps, which now are often small enough to fit into a seat pack or even a back pocket - sometimes a classic frame-fit pump is a great way to go. And when it comes to frame-fit pumps, the most common choices have long been either the Zefal or the Silca. I've used both, and honestly the Zefal works a little better - but the Silca is my sentimental favorite.

The classic plastic-bodied Silca Impero has long been discontinued, but lightly used or even NOS examples are still easy to find. Getting them in the right length, however, can sometimes be the trick. Not all that long ago, there was a discussion on the Classic Rendezvous group about shortening a Silca frame pump. The timing was pretty fortunate, since I had recently picked up a nice old pump on eBay that turned out to be just about an inch too long for any of my bikes (always make sure about whether the seller's measurement includes the pump head or not!). Since I didn't really want to bother with the hassle of trying to return the pump for a refund, I decided to look into shortening it instead.

There are a couple of methods for shortening a pump. The method that the CR friends seemed to agree was the most reliable is pretty clever, and seemed easy enough to follow, but unfortunately, also requires a drill press for the best results, which I don't happen to own or have access to. However, I did find someone in the group who was kind enough to shorten the pump for me, which was great. Still, even though I didn't end up doing it myself, I thought that revealing the method might prove useful to some Retrogrouch readers who might want to give it a try.

So here, with some pretty simple diagrams, is the method for shortening a Silca frame pump:

First, you'll need to disassemble the pump. Easy enough to do. Remove the plunger, and take off the head, rubber gaskets, and metal sleeve.

Remove the plunger by unscrewing the knurled ring at the top of the barrel.

Unscrew the pump head and remove the gasket and the metal sleeve. Cutting operation will begin by removing material at this end of the pump, and then re-gluing the nipple end back into the shortened plastic barrel.

From here on, you'll have to refer to my home-made diagrams:

Carefully cut the pump to the desired length from the nipple end of the shaft as shown. Make sure that the cut is straight, and clean up any burrs on the barrel with fine sandpaper.

To make it easier to work with for the next step, you may want to make another cut to remove some more of the excess material. Get it down to roughly one inch or a little less.

Here's where the drill press comes in. Carefully chuck the nipple end into the drill press. Take precautions not to damage the threads. The drill press will essentially work like a lathe.

With the end of the pump spinning in the drill press, and using a file, remove the remaining bit of the barrel tube from the "plug" that is glued into the end. Work slowly, and remove just enough material to reveal the plug. You can always take off a little more, but you can't put it back on once it's gone. When it's finished, it should just slip into the newly cut barrel tube.

Use 2-part epoxy to re-glue the plug into the newly shortened pump barrel. Make sure everything is clean and grease-free before gluing.

Next, you need to shorten the plunger tube by the same amount.

Again, cut straight, and make sure the cut is clean on the piece you're saving.

The plunger itself is simply pressed into the shaft and held in place by a couple little punched-in "dimples." Carefully put the plunger shaft into a vise and file through the aluminum shaft to free the plug.

Press the plunger piece into the newly shortened shaft.

Tap with a center punch tool to make new dimples to hold the plunger piece in place. Really - that's how they did it a the factory. If you want to add some epoxy in there just to be safe, it probably won't hurt, but it will make any future changes more difficult.

As you can see, the process seems pretty straightforward. The only thing that prevented me from trying it is the lack of a drill press. If you have a Silca Impero that is too long, and you have the tools, it might be worth a try.

I've done this for years, but didn't want to broadcast it, in a selfish attempt to keep the competition for old Silca pumps from increasing. I use a lathe to turn the plastic barrel off the plug end. There's one key step missing from your instructions. Before doing any cutting, fit your chosen pump head with a fresh washer, but without the aluminum grip. You can cut washers from thick cork gasket material, available at auto parts stores in small rolls. If you cannot find any cork of suitable thickness, cut two washers and use them together. Tighten the head fully and then make a mark on the edge of the plug where the front of the pump should be. When you glue the plug back into the barrel, use the mark to locate the Silca logo so that it will display correctly. Your modified pump may just be better than a stock one!

If you've never used a Silca frame pump before, know that they got a bad rep back in the day, as they can "explode" if you're not careful. There is no check valve in the pump, so if you push it onto a presta valve and it contacts the valve release as you are doing so, the pump fills up with the pressurized air from the tube and it can send the plunger off into the air like a rocket. I once watched one clear a two-lane highway and end up in a field on the other side. The trick is to hold the end of the pump in the inside crook of your elbow, so that if the pump pressurizes, your arm blocks the handle from extending. It also signals you that you need to pay more attention to pushing the head straight onto the valve. A little spit on the valve stem works wonders. Functionally speaking, a Zefal with its thumb lock is a better pump, but it is heavier, and has its own quirks.

I've never seen one launch the plunger as you describe, but I have heard of it happening. And I did have one come apart once when the top of the pump cracked slightly (where the knurled plunger ring threads in), so that it wouldn't hold pressure.

I agree with you that the Zefal with the thumb lock works a little better. But as I said, the Silca is a sentimental favorite.

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The Retrogrouch

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Although I certainly didn't coin the term "Retrogrouch," the name sums up my attitudes on bicycles. I'm incredibly opinionated on bikes, and really believe that they are best when they are beautiful, reliable, and simple (not necessarily in that order).